Spectrophotometer utilizing memory storage calibration techniques

ABSTRACT

A spectrophotometer having no moving parts in the monochromator or the optical beam paths through the reference material and the sample material is disclosed. A preferred form of monochromator utilized is an acousto-optical filter tuned over the desired spectrum electronically by variation of the radio frequency producing the acoustical wave in the filter. A single beam spectrophotometer employs a memory device for storing the detected transmitted optical radiation in a first calibration run with the sample absent from the path, a divider circuit producing a ratio signal output responsive to one input comprising the detected transmitted optical radiation in a second measurement run with the sample in place and a second input comprising the absorption signal stored in the memory. A double beam spectrophotometer utilizes an optical beam splitter to split the radiation into two separate paths, one path extending through a reference material and the other path through the sample, the transmitted light detected in the two paths being transmitted to a divider circuit to produce an output dependent on the ratio of the two detected radiation beams. A calibration run with reference and sample absent may be made and stored in a memory, followed by a measurement run with reference and sample in place, and the two output ratios then compared to give a final ratio signal output. In a further embodiment, two optical sources and two associated monochromators are provided, the radiation from each being directed through the beam splitter into the two separate reference and sample paths to the detector circuitry.

E t [H] oody et al. 22, 1973 SPECTROPHOTOMETER UTILEZHNG 1 ABSTRACTMEMORY STORAGE CALKBTKGN TECHNKQUES A spectrophotometer having no movingparts in the [75] Inventors. Robert E. Moudy, Los Altos Hills;monochromator or the optical beam paths through the David A. Wilson PaloAlto, both of reference material and the sample materlal s dis- Calificlosed. A preferred form of monochromator utihzed is an acousto-opticalfilter tuned over the desired spec- [73] Assignee: Hewlett-PackardCompany, Palo trum electronically by variation of the radio frequency AlC lif, producing the acoustical wave in the filter. A single beam sectro hotometer em 10 s a memor device [22] led: 1971 for storing thgdetected trans init ted optical radiation 21 Appl 205 243 in a firstcalibration run with the sample absent from the path, a divider circuitproducing a ratio signal output responsive to one input comprising thedetected US. Cl. t transmitted ptical radiation in a econd measurement[51] Int. Cl 3/42 un with the ample in place and a econd input com- [58]Field of Search ..356/5l, 88, 89, 93-97, prising the absorption signalstored in the memory. A

356/204-206,229;350/149 double beam spectrophotometer utilizes anoptical beam splitter to split the radiation into two separate [56]References Ci d paths, one path extending through a reference materialand the other path through the sample, the trans- UNITED STATES PATENTSmitted light detected in the two paths being transmitted to a dividercircuit to produce an output dependent on the ratio of the two detectedradiation beams. A calibration run with reference and sample FOREIGN NTSO C O S absent may be made and stored in a memory, followed by ameasurement run with reference and sample in 1,074,810 7/1967 GreatBritain ..356/229 place, d th t t t ti th compared t OTHER PUBLICATIONSgive a final ratio signal output. In a further embodi- Chance et al.,The Review of Scientific Instruments, Vol. 41, No.1, January 1970, Pagesill-I15.

Primary ExaminerRonald L. Wibert Assistant ExaminerF. L. EvansAtt0rneyRoland I. Griffin ment, two optical sources and two associatedmonochromators are provided, the radiation from each being directedthrough the beam splitter into the two separate reference and samplepaths to the detector circuitry.

12 Claims, 5 Drawing Figures ,i3 PROGRAM SCAN 27 r I 42 44 26 *3 MODTUNABLE TllNABLE MOD j DWIDKR 3 RFGEN. arm J4 WI m2 DIVIDER 1 1 J L r rJ 28 PATENIEL 111.! 2 21975 SCAN SHEET 1 BF 3 H I2 14 I5 I ul6 A? MEMORYOPTICAL OPTICAL W SO r-fiMONOCHROMETER SAMPLE DETECTOR I I I J 13 19 r yy 18 PROGRAM RECORDER DIVIDER igure 1 PROGRAM RECORDER DISPLAY MEMORYSPECTROPHOTOMETER UTILIZING MEMORY STORAGE CALIBRATION TECIWIQUESBACKGROUND OF THE INVENTION A number of different types ofspectrophotometers wherein a sample is chemically and/or physicallyanalyzed by measuring the absorption of optical radiation passingtherethrough as a function of the optical wavelength are presently inuse. A typical form of spectrophotometer will scan through a spectrum,from 2,000 to 8,000 A, with a resolution, for example, of l to A. In arelatively simple type of single beam system, the radiation from asource of light is directed through a lens system and into amonochromator, typically an optical prism system or diffraction grating,which serves to separate the narrow band of wavelengths to be deliveredto the sample under investigation. Generally, mechanical means such as amoving disperser serves to scan the monochromator output past a slit forthe full spectrum range of interest. An optical detector such as aphotomultiplier device measures the light passing out from the sample,the percentage of light transmission being recorded on a suitablerecording means such as an X-Y recorder as a function of the scannedradiation. This plot of percent optical transmission is then used toanalyze the sample.

These spectrophotometers are divided into various general classes inaccordance with specific structural characteristics. For example,spectrophotometers may utilize optical prisms, diffraction gratings,combination prism-grating systems, or combination filter-grating systemsfor providing the narrow line spectrum output from the monochromator.Spectrophotometers may also function as single beam or double beamsystems, the double beam systems being sub-classified in accordance withwhether the optical beam is divided in intensity or time multiplexed.

Considering the single beam device, the signal output is a product ofthe following general factors, all of which are a function of thewavelength; the power output of the light source, PO), the efficiency oflight transmission of the monochromator, and associated optics E()\),the transmission efficiency of the sample material, T()t), and thesensitivity of the light detector, 8(a). Thus, although it is desiredthat only the transmission efficiency of the sample material be measuredas a function of the scanned radiation, the other three factorscontribute to the output obtained from the system. To compensate forthese undesired factors in the output, prior art devices have sought toprecalibrate the system, the operator being instructed to take intoaccount the calibration of the instrument across the spectrum inanalyzing the output signal.

To overcome this requirement for calibration on the part of theoperator, double beam systems are utilized wherein the optical radiationis transmitted over two separate paths, one path including the samplematerial under analysis and the other path including a referencematerial. The output radiation from the two detectors are compared sothat the final output is the ratio, T /T of the light transmissiondetected in the sample path, T and the light transmission detected inthe reference path, T

In one form of double beam system the output from the monochromator istransmitted to a beam splitter where the optical radiation is dividedinto two separate paths, one optical beam being transmitted through thereference material and the other optical beam being transmitted throughthe sample material under investigation. The radiation output from thereference material is transmitted to a first optical detector and theradiation from the sample material is transmitted to a second opticaldetector. The outputs from the two optical detectors are transmitted toan electronic divider circuit which produces an output proportional tothe ratio of T to T as a function of A. In this dividing step, thevarious undesired factors which are functions of the wavelengthmentioned above cancel out except for differences of sensitivity of theoptical detectors. This type of double beam system has the advantagethat there are no moving parts in the radiation beam path from themonochromator to the optical detectors. However, this system issensitive to unbalance in the division of the radiation by the splitterinto the two paths, and photomultiplier sensitivity differences. Forexample, one path may contain percent red and 40 percent blue lightwhereas the other path contains an inverse ratio. In addition, it isnecessary that the optical detectorsbe very closely matched. Also, thissystem suffers from the fact that the optical beam passing through thesample under analysis is only one-half of the total radiation emittedfrom the source.

To avoid the problems encountered with transmission of only half theradiation through the sample material and the requirement of a matchedpair of light detectors, a different form of double beam instrument isutilized wherein the optical radiation is time mu1ti plexed such thatfor one portion of a time interval the radiation passes through thereference material and for the other portion of the time interval theradiation passes through the sample material under analysis. Theradiation output from the monochromator is directed over the twoseparate paths by a rotating mirror chopper, the output radiation fromthe reference and sample being directed by a second rotating mirrorsynchronized with the first mirror onto a single optical detector. Theoutput from the single detector is then sent to suitable electroniccircuitry where the ratio of T to T R is computed. This type of systemis rather complex in that it requires moving parts in the two beam pathsand the instrument is relatively slow because of the mechanicalarrangement. In addition, the mirrors must be very carefully matched toobtain equal reflectivity over the entire wavelength range scanned.

SUMMARY OF THE INVENTION In the present invention a novelspectrophotometer is provided which has no moving parts, is extremelyfast, and does not require calibration computation by the operator aftereach sample analysis.

In one embodiment of the present invention, the monochromator output isscanned in a narrow waveband increment, i.e., l to 10 A, over a wideradiation spectrum. In a first calibration run, the radiation outputfrom the monochromator is transmitted to a suitable radiation detectorsuch as a photomultiplier with the sample material to be analyzed absentfrom the optical path. The output of the radiation detector is stored ina memory system scanned in synchronism with the scan of themonochromator. As a result, the radiation transmission characteristic ofthe system, absent the sample material, is recorded in the memory.Thereafter, with a sample to be analyzed, positioned in the radiationpath during a subsequent scan by the monochromator,

the output from the radiation detector is transmitted to a dividercircuit, the other input to the divider circuit being the calibrationsignal stored in the memory and delivered in synchronism with the scanof the monochromator. The output of the divider circuit comprises asignal related to the ratio of the detected light transmission with thesample to the light transmission without the sample in place.

In another embodiment of the present invention, the optical radiationfrom the monochromator is divided by a beam splitter into two separatepaths, one optical path passing through a reference material and theother optical path extending through the sample material under analysis.Anoptical detector at the end of the reference path transmits its signaloutput to a divider circuit, said divider receiving its other input fromthe optical detector in the sample path; the signal output of thedivider is related to the ratio of the detected optical transmission inthe two paths. In a first run, the reference and the sample are omittedfrom the two paths and the optical transmission ratio in the two pathsis recorded in the memory as a function of the spectrum scan. In asubsequent scan, with both the reference and the sample being positionedin their respective optical beam paths, the outputs from the tworadiation detectors are transmitted to a divider circuit for producingan output signal related to the ratio of the radiation detected in thetwo paths. The signal output from the divider is then compared in asecond divider with the output from the memory for the correspondingwavelength to give a resultant output related to the ratio of the tworatios.

In additional embodiments of the invention, two optical sources and twoassociated monochromators are employed, each providing a portion oftheir radiation output to the two optical paths through the referenceand the sample. In certain instances the two separate radiation beamsmay be at the same wavelengths and in other operations the two may be ofdifferent wavelengths.

A preferred form of monochromator for use in these embodiments comprisesan acousto-optic filter which is electronically tuned across the desiredradiation spectrum, the memory being synchronized with the filter scan.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a single beamspectrophotometer apparatus of the present invention.

FIG. 2 is a block diagram of a double beam spectrometer of the presentinvention.

FIG. 3 is a schematic diagram illustrating a portion of the system ofFIG. 2.

FIG. 4 is a block diagram of a modification of the double beamspectrophotometer of FIG. 2 to provide an increased radiation spectrumrange.

FIG. 5 is a block diagram of still another two beam embodiment of thepresent invention providing two separate simultaneous analyses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thespectrophotometer comprises a radiation source 11 having a wide bandemission spectrum and a monochromator 12 for selecting a narrowwavelength increment, e.g., AA l A to A, with programmer means 13 toscan the narrow band monochromator output over the desired radiationspectrum, e.g., from 2,000 A to 8,000 A, or from 25pm to 15pm.

The monochromator output is directed along a path through a sample 14under analysis and onto an optical detector 15 that produces an outputsignal dependent on the impinging radiation. A switch 16, while in oneposition, couples the output of the optical detector to a memory device17, such as a digital storage system, which is also coupled to thescanning means 13. In the second position, the switch 16 couples theoptical detector output to a divider circuit 18, the other input of thedivider being coupled to the output of the memory 17. The output of thedivider 18 is coupled to a suitable recording means such as an X-Yrecorder 19.

In operation, a first scan through the radiation spectrum is instigatedwith a known reference material in the optical path between themonochromator l2 and the optical detector 15, and with the switch 16connecting the output of the optical detector 15 to the memory 17. Thememory is scanned in syn-chronism with the monochromator, and the outputof the optical detector, which is related to the optical radiationimpinging thereon, is stored in the memory as a function of the scannedspectrum.

Thereafter, the sample to be analyzed replaces the reference sample inthe optical path, and a new scan of the radiation spectrum is initiatedby the programmer 13, with the switch 16 connecting the optical detector15 output to the divider circuit 18. The stored reference signal in thememory 17 is delivered to the divider 18 in synchronism with the scannedradiation, and the divider operates to produce an output signal which isrelated to the ratio of the detected radiation transmission during thesample run, T to the detected radiation transmission during thereference run, T This output ratio signal is recorded on the X-Yrecorder which is synchronized with the radiation scan.

In a preferred form of spectrophotometer, the monochromator comprises anacousto-optic filter tunable over its spectrum range electronically. Theoptical frequency of the output radiation of the filter is tuned over arelatively wide band of optical frequencies, for example, 2,000 A to4,000 A in the case of a quartz filter and 4,000 A to 8,000 A in thecase of a CaMoO crystal, by varying the frequency of an acoustical wavecollinearly disposed with the optical wave in the crystal. The acousticwave is tuned by means of a transducer attached to the crystal andactivated from a tunable radio frequency source. Such an acousto-optictunable filter is disclosed in an article entitled Acousto-Optic TunableFilter in the Journal of the Optical Society of America, Vol. 59, No. 6,June 1969, pages 744-747, and an article entitled Electronically TunableAcousto-Optic Filter in the Applied Physics Letters, Vol. 15, No. 10,Nov. 15, 1969, pages 325 and 326. As the acousto-optic filtermonochromator is scanned over its operating band width by tuning theacoustic wave, the memory 17 is scanned in synchronism therewith, as isthe X-Y recorder 18.

The use of an acousto-optical filter is advantageous for a number ofreasons. Since the filter is electronically tunable, the system hasrandom access capability; specific segments within the overall spectrumrange can be easily selected, and switching from one discrete frequencyrange to another is readily accomplished electronically.

In prior art monochromator systems, all the light, including theundesired scattered radiation, was chopped, and the undesired light wasalso detected since it was at the correct chopped frequency, and anerror signal resulted. Since only the desired wavelengths of light fromthe acousto-optic filter are chopped, unwanted wavelengths of light canbe eliminated easily by electronic techniques.

Most monochromators have long optical paths leading to errors due toatmospheric absorption. The acousto-optical filter system has arelatively short optical path. Also, no optical coatings such asaluminum reflectors which tend to age and introduce errors into thesystem are utilized.

A double beam embodiment of the invention is disclosed in FIG. 2 andincludes the optical source 1 1, and lens system 11 if needed,delivering the wide band optical radiation to the monochromator 12,preferably an acousto-optic filter which is scanned over its operatingwavelength range by tuning the radio frequency generator 12' undercontrol of the scan unit 13.

The output of the filter 12 is split into two separate paths by anoptical beam splitter 21, for example an uncoated synthetic fused silicaor fused quartz plate, one beam path 22 being directed through areference mate rial region and the other beam path 23 being directedthrough a sample material region. A satisfactory beam splitting isobtained with the synthetic fused silica plate when the incident angleis about 72; an equal splitting of the optical radiation is preferredbut not necessary.

The reference beam path 22 terminates in a suitable optical detector 24,for example a photomultiplier, and the sample beam path terminates in asimilar optical detector 25. The outputs from the two optical detectorsare coupled to a divider circuit 26 which produces an output which isthe ratio of the radiation detected in the sample path to that detectedin the reference path.

The output of the divider 26 is coupled via one position of switch 27 toa memory device 28, such as a digital storage apparatus, and coupled viaa second switch position to another divider circuit 29. The other inputto the divider circuit 29 is the output from the memory 28. The outputof the divider 29 is coupled via switch 31 to a second memory apparatus32 or to a recorder display device 33.

In operation a first scan of the spectrophotometer is made with thereference material 34 and the sample material absent from their beampaths 22 and 23, respectively, and with the switch 27 programmed toclose the output of the divider 26 to the memory device 28. Thiscalibration run may be conducted the first thing in the morning, forexample, or as often during the day as the operator desires arecalibration.

The output of the divider circuit 26 will be the ratio of the detectedoptical radiation transmission in the sample path to the radiation inthe reference path. The ratio will be close to l, and will vary acrossthe spectrum scan of the system, and it will be a function ofreflectivity and transmission of the quartz plate and of the quantumefficiencies of the two photomultipliers. If the beam splitter sentexactly equal amounts of radiation along each path, and if bothphotodetectors had identical sensitivity, at each wavelength, then theratio stored in the memory would be 1 at each wavelength. This ratio is:

where I is the light intensity from the filter, B is the transmission ofthe plate 21, a is the reflectivity from the plate 21, and Q and Q, arethe quantum efficiencies of the two photodetectors 24 and 25,respectively. The radiation intensity I ()t, t) cancels out in theratio.

The next and subsequent runs are conducted with a reference material inthe reference path and a sample material in the sample path. The switch27 is operated to close the output of the divider circuit 26 to thesecond divider 29. The output from the divider circuit 26 is a new ratioR which has added information as to the transmission of the twomaterials, T and T This new ratio is:

2 0% 31 Q2 s/ M0 1 Q1 a] [31 Q2 S l Q1 TR] During the scan time when theoutput of divider 26 is transmitted to divider 29, the memory 28 isoperated in synchronism with the scan to deliver the ratio storedtherein to the divider 29 which operates on these two input ratios R 1and R to produce a new output ratio R Thus 3 e/ 1) (B1 Q2 s/ i Q1TR)/(B1 Q2 1 Q1) (TS/TR) It can be seen that all factors drop out exceptfor T and T and this resultant ratio serves as a very accuratemeasurement of the sample under analysis.

This ratio may be stored in a second memory 32 or it may be recorded ona suitable display means such as an X-Y recorder. If the ratio stored inmemory 28 need not be used more than once, the memory 28 may serve theplace of memory 32 to store the new ratio as the stored ratio is removedfrom the memory 28.

It is desired that the sensitivity of the two optical detectors maintaina fixed relationship since changes in the relative sensitivities willproduce errors in the output from the system. The two optical detectorsare also operated in their linear regions for best results. To helpmaintain the fixed relationship in sensitivity, the two detectors 24 and25 are operated from the same power supply 36. It is also preferablethat the two detectors have a common set of resistors establishing thedynode potentials as illustrated in FIG. 3.

To broaden the range of the system, a second optical source 37, lenssystem 37 and monochromator or acousto-optic filter 38 may be added tothe system as shown in FIG. 4. If filter 12 is a quartz filter with adeuterium or UV source 11 covering the optical range of 2,000 A to 4,000A, then the additional range of 4,000 A to 8,000 A may be covered by asecond filter 38 of a CaMoO crystal and a tungsten or visible lampsource 37. The program scan circuit will first operate source 11 andfilter 12 for the shorter wavelength spectrum scan and will then switchto operation of source 37 and filter 38 for the subsequent longerwavelength spectrum scan. The system may also be operated with the twolight sources 11 and 37 on at the same time.

The system of FIG. 4 may be used in a different mode of operation withmonochromators 12 and 38 being similar and operating over the wholewavelength range covered by optical sources 11 and 37. In this mode bothmonochromators are operated simultaneously with both optical sourcescontributing to the signal strength simultaneously although thewavelength regions of maximum light output differ for the two lamps.

Where a higher intensity of light is desired through the two materials,the optical source 11 and monochromator 12 may be similar to opticalsource 37 and monochromator 38, the sources being operatedsimultaneously and the monochromators being scanned together, resultingin a substantially doubled light intensity through the system.

In the embodiment of the invention shown in FIG. 5, the two separatelight beams are at two separate wavelengths, for example, 3,700 A and4,000 A, so that an analysis of the sample may be carried out at the twodis crete wavelengths simultaneously. One of the acoustooptic filters 12is operated at an RF frequency of m, from RF generator 41 (i.e., 100 MHzto give 3,700 A) and the other filter 38 is operated at an RF frequencyof to: from RF generator 42 (i.e., 90 MHz to give 4,000 A); the mfrequency is modulated by modulator 43 at m for example 1 KHZ and the mfrequency is modu lated by modulator 44 at for example l.7 KHZ.

Both modulated wavelengths pass through the reference 34 and the sample35 to the associated optical detectors 24 and 25, respectively. A pairof electronic filters 45 and 46 centered at (0 and respectively, arecoupled to the output of detector 24 and a second pair of similarelectronic filters 47 and 48 are coupled to the output of detector 25.The outputs of the two (0 filters 45 and 47 are coupled to one divider26 and pass the 3,700 A light from the two paths to the divider and itsassociated measurement circuitry 27-33. The outputs of the two m filters46 and 48 are coupled to a second divider 26' and pass the 4,000 A fromthe two paths to this divider and its associated measurement circuitry27'-33'.

Both monochromators can sit on their respective wavelengths for the twoseparate measurements to be made, or both can be scanned by the programscan 13 as described above, or one can remain fixed and the otherscanned if desired.

Although quartz and calcium molybdate crystals have been suggested foruse as the acousto optical filters in certain of the above embodiments,it should be understood that other known crystals may be employed suchas lithium niobate and proustite. It should also be noted that thetechniques described in these monochromator systems are applicable overa wide optical range including the infrared, visible, and ultraviolet,and can extend from 1,000 A to 1,000 microns. Various types of opticalsources and optical detectors, including solid state devices, may beemployed. The systems can be made in a modular design so that sourcesand/or detectors can be replaced to extend the range of the device overdifferent frequency bands, for example an ultraviolet device with add-onunits for visible and infrared operation.

We claim:

1. Apparatus for analyzing a sample material comprising:

a first source for emitting a first beam of optical radiation having afirst emission spectrum;

a second source for emitting a second beam of optical radiation having asecond emission spectrum;

a first monochromator for selecting a narrow band of radiation from thefirst beam and scanning it over the first emission spectrum;

a second monochromator for selecting a narrow band of radiation from thesecond beam and scanning it over the second emission spectrum;

beam splitting means for splitting each of the narrow bands of radiationinto two portions and directing a first portion of the radiation fromthe first beam simultaneously with a first portion of the radiation fromthe second beam along a reference path normally containing a referencematerial and directing a second portion of the radiation from the firstbeam simultaneously with a second portion of the radiation from thesecond beam along a sample path normally containing a sample material tobe analyzed;

first detection means for detecting the radiation traversing the samplepath to develop an output signal related to the radiation impinging onthe first detection means;

second detection means for detecting the radiation traversing thereference path to develop an output signal related to the radiationimpinging on the second detection means;

first comparator means for comparing the output signals obtained fromthe first and second detection means during a first emission spectrumscan with the reference and sample materials absent from theirrespective paths, to produce a first ratio signal indicative of theratio of the output signals;

memory means for storing the first ratio signal as a function of thefirst emission spectrum scan;

second comparator means for comparing the first ratio signal stored inthe memory means with a second ratio signal obtained from the firstcomparator means during a second emission spectrum scan with thereference and sample materials present in their respective paths toobtain an output signal related to the ratio of the first and secondratio signals as a function of the first and second emission spectrumscans.

2. Apparatus as in claim 1 wherein said first and second monochromatorsselect different narrow bands of radiation.

3. Apparatus as in claim 2 wherein said first and second sources havedifferent emission spectra.

4. Apparatus as in claim 2 wherein said first and second sources haveidentical emission spectra.

5. Apparatus as in claim 1 wherein said first and second monochromatorsselect identical narrow bands of radiation.

6. Apparatus as in claim 5 wherein said first and second sources havedifferent emission spectra.

7. Apparatus as in claim 5 wherein said first and second sources haveidentical emission spectra.

8. Apparatus as in claim 1 wherein each of said first and secondmonochromators comprises an acoustooptical filter and means forelectronically tuning the filter over one of the first and secondemission spectra.

9. Apparatus as in claim 8 wherein each of said filters is made of amaterial selected from the group consisting of quartz, calciummolybdate, lithium niobate and proustite.

10. Apparatus as in claim 1 wherein said beam splitting means comprisesan optical plate for transmitting a portion of each beam and reflectinga portion of each beam.

11. Apparatus as in claim 1 including:

9 10 modulating means for electronically modulating the nal that isresponsive to the radiation from the secradiation from each source; andnd source demodulating means for electronically demodulating 11Apparatus as in claim 11 wherein Said modulating the output signals fromthe first and second detection means in order to distinguish a part ofeach means frequency modulates the radlatlon from each output signalthat is responsive to the radiation Sourcefrom the first source and apart of each output sig-

1. Apparatus for analyzing a sample material comprising: a first sourcefor emitting a first beam of optical radiation having a first emissionspectrum; a second source for emitting a second beam of opticalradiation having a second emission spectrum; a first monochromator forselecting a narrow band of radiation from the first beam and scanning itover the first emission spectrum; a second monochromator for selecting anarrow band of radiation from the second beam and scanning it over thesecond emission spectrum; beam splitting means for splitting each of thenarrow bands of radiation into two portions and directing a firstportion of the radiation from the first beam simultaneously with a firstportion of the radiation from the second beam along a reference pathnormally containing a reference material and directing a second portionof the radiation from the first beam simultaneously with a secondportion of the radiation from the second beam along a sample pathnormally containing a sample material to be analyzed; first detectionmeans for detecting the radiation traversing the sample path to developan output signal related to the radiation impinging on the firstdetection means; second detection means for detecting the radiationtraversing the reference path to develop an output signal related to theradiation impinging on the second detection means; first comparaTormeans for comparing the output signals obtained from the first andsecond detection means during a first emission spectrum scan with thereference and sample materials absent from their respective paths, toproduce a first ratio signal indicative of the ratio of the outputsignals; memory means for storing the first ratio signal as a functionof the first emission spectrum scan; second comparator means forcomparing the first ratio signal stored in the memory means with asecond ratio signal obtained from the first comparator means during asecond emission spectrum scan with the reference and sample materialspresent in their respective paths to obtain an output signal related tothe ratio of the first and second ratio signals as a function of thefirst and second emission spectrum scans.
 2. Apparatus as in claim 1wherein said first and second monochromators select different narrowbands of radiation.
 3. Apparatus as in claim 2 wherein said first andsecond sources have different emission spectra.
 4. Apparatus as in claim2 wherein said first and second sources have identical emission spectra.5. Apparatus as in claim 1 wherein said first and second monochromatorsselect identical narrow bands of radiation.
 6. Apparatus as in claim 5wherein said first and second sources have different emission spectra.7. Apparatus as in claim 5 wherein said first and second sources haveidentical emission spectra.
 8. Apparatus as in claim 1 wherein each ofsaid first and second monochromators comprises an acousto-optical filterand means for electronically tuning the filter over one of the first andsecond emission spectra.
 9. Apparatus as in claim 8 wherein each of saidfilters is made of a material selected from the group consisting ofquartz, calcium molybdate, lithium niobate and proustite.
 10. Apparatusas in claim 1 wherein said beam splitting means comprises an opticalplate for transmitting a portion of each beam and reflecting a portionof each beam.
 11. Apparatus as in claim 1 including: modulating meansfor electronically modulating the radiation from each source; anddemodulating means for electronically demodulating the output signalsfrom the first and second detection means in order to distinguish a partof each output signal that is responsive to the radiation from the firstsource and a part of each output signal that is responsive to theradiation from the second source.
 12. Apparatus as in claim 11 whereinsaid modulating means frequency modulates the radiation from eachsource.